1. Field of the Invention
The present invention relates to a system and method for performing periodic ranging in sleep mode in a communication system.
2. Description of the Related Art
In general, future-generation communication systems are being developed to provide high-speed large-data transmission/reception service to Mobile Stations (MSs). A major example of such systems is Institute of Electrical and Electronics Engineers (IEEE) 802.16e.
FIG. 1 illustrates the configuration of a conventional IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system is configured in a multi-cell structure, including cells 100 and 150, a Base Station (BS) 110 covering the cell 100, a BS 140 covering the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153.
As the IEEE 802.16e communication system supports Mobile Station(MS)'s mobility, the power consumption of the MS is a significant factor that affects overall system performance. To minimize the MS's power consumption, a sleep mode and an awake mode operation between an MS and a BS have been proposed. The MS periodically compensates for timing and frequency offsets, and transmits power with respect to the BS in order to handle changes in the channel status between the MS and the BS. This operation is called ranging, and it includes initial, periodic and bandwidth request ranging. In light of the support of the MS's mobility, periodic ranging is rendered more significant.
FIG. 2 illustrates a periodic ranging operation in sleep mode in the conventional IEEE 802.16e communication system.
Referring to FIG. 2, an MS 250 transmits a SLeeP-REQuest (SLP-REQ) message to a BS 200 to transition from awake mode to sleep mode in step 201. The BS 200 determines whether to approve the awake-to-sleep mode transition, considering its status and the status of the MS 250 and transmits a SLeeP-ReSPonse (SLP-RSP) message corresponding to the determination result to the MS 250 in step 203. The SLP-RSP message contains a Next Periodic Ranging parameter. Upon receipt of the SLP-RSP message, the MS 250 detects the Next Periodic Ranging parameter, based upon which it prepares for periodic ranging. The Next Periodic Ranging Parameter is an offset indicating when the MS 250 is to wake up from the sleep mode in order to be allocated an uplink burst for periodic ranging from the BS 200. The offset is assumed to be a frame offset herein. In a frame indicated by the Next Periodic Ranging parameter, counted from the frame carrying the SLP-RSP message in a sleep interval, the MS 250 transitions from the sleep mode to the awake mode to perform the periodic ranging. If the MS 250 is already in the awake mode, it only has to perform the periodic ranging in the frame. If the sleep interval still lasts after the periodic ranging, the MS 250 may return to the sleep mode.
Meanwhile, the MS 250 performs the sleep mode operation, while increasing the sleep interval by an algorithm. If the MS 250 is in the sleep mode in the frame designated for periodic ranging by the Next Periodic Ranging parameter, it transitions to the awake mode in step 205 in which it performs the periodic ranging in step 207.
The periodic ranging is performed through at least one exchange of a RaNGing REQuest (RNG-REQ) message and a RaNGing ReSPonse (RNG-RSP) message in steps 211 to 225. When the MS 250 gets the uplink burst for the periodic ranging corresponding to the Next Periodic Ranging parameter from the BS 200 in step 205, it transmits an RNG-REQ message to the BS 200 in the allocated uplink burst in step 211. The BS 200 transmits ranging response information with frequency, time, and transmit power to be compensated for to the MS 250 by an RNG-RSP message in step 213. If the frequency, time and transmit power need to be additionally compensated for, Ranging Status is set to 1 indicating Continue.
Upon receipt of the RNG-RSP message with Ranging Status set to 1, the MS 250 detects parameters necessary for compensating for the frequency, time and transmit power from the received message and performs the compensation. In step 215, the MS 250 transmits an RNG-REQ message to the BS 200 in order to continue the on-going frequency, time and transmit power compensation.
Upon receipt of the RNG-REQ message, the BS 200 performs the periodic ranging by repeating the RNG-REQ/RNG-RSP exchange in steps 217, 219, 221 and 223. If it determines that no further frequency, time and transmit power compensation is needed, the BS 200 transmits to the MS 250 an RNG-RSP message with Ranging Status set to 3 indicating Success and a Next Periodic Ranging parameter indicating a frame for the next periodic ranging in step 225.
Upon receipt of the RNG-RSP message with Ranging Status set to 3 and the Next Periodic Ranging parameter, the MS 250 prepares for the Next Periodic Ranging in the frame indicated by the Next Periodic Ranging parameter, considering that the periodic ranging has been completed. If a sleep interval still lasts after the periodic ranging is completed, the MS 250 may return to the sleep mode.
When the MS 250 is in the sleep mode in the frame indicated by the Next Periodic Ranging, it transitions to the awake mode and performs the periodic ranging. When the MS 250 is in the awake mode in the frame indicated by the Next Periodic Ranging, it just performs the periodic ranging in the awake mode. To be more specific, when the MS 250 is in the awake mode at the start of periodic ranging, it naturally decodes a DownLink-MAP (DL-MAP) message or an UpLink-MAP (UL-MAP) message to monitor the presence or absence of a data burst allocated to the MS 250 in a downlink frame. If the BS 200 has given a periodic ranging opportunity to the MS 250, that is, the BS 200 has allocated an uplink burst to the MS 250, the MS 250 is aware of the periodic ranging opportunity.
On the other hand, if the MS 250 is in the sleep mode at the start of the periodic ranging, it transitions to the awake mode and decodes the DL-MAP or UL-MAP message to monitor the presence or absence of a data burst allocated to the MS 250 in the downlink frame.
As described above, the Next Periodic Ranging parameter applies irrespective of whether the MS 250 is in the sleep mode or the awake mode until just before or at the start of periodic ranging. This is described as a “Don't care about sleep and normal operation” in step 227. That is, it can be said that the sleep-mode periodic ranging operation is performed considering both the sleep mode and the periodic ranging, while ensuring maximal compatibility with the conventional IEEE 802.16e communication system.
While not described specifically in relation to step 227, it is to be noted that the MS 250 should re-calculate a frame in which to transition to the awake mode according to the latest Next Periodic Ranging parameter received in an SLP-RSP or RNG-RSP message. For example, the MS 250 transitions to the awake mode and then returns to the sleep mode by receiving an SLP-RSP message during step 227. Thus the MS 250 determines when to transition to the awake mode using a Next Periodic Ranging parameter included in the SLP-RSP message.
FIG. 3 illustrates a signal flow for the periodic ranging operation in the sleep mode between the MS and the BS in the conventional IEEE 802.16e communication system.
Referring to FIG. 3, to transition from the awake mode to the sleep mode, an MS 300 transmits an SLP-REQ message to a BS 350 in step 311. The BS 350 determines whether to approve the awake-to-sleep mode transition according to its status and the status of the MS 300 and transmits an SLP-RSP message according to the determination result to the MS 300 in step 313. The SLP-RSP message contains a Next Periodic Ranging parameter. Upon receipt of the SLP-RSP message, the MS 300 starts a sleep mode operation according to the SLP-RSP message in step 315. The MS 300 is also aware of when to perform periodic ranging from the Next Periodic Ranging parameter.
During the sleep mode operation, the MS 300 transitions to the awake mode in a frame indicated by the Next Periodic Ranging parameter, for periodic ranging in step 317. In step 323, the MS 300 detects a periodic ranging opportunity, i.e. an uplink burst allocated to the MS 300 from a UL-MAP message broadcast by the BS 350. The MS 300 transmits an RNG-REQ message to the BS 350 in the uplink burst in step 325.
The BS 350 transmits information needed for compensating for frequency, time and transmit power to the MS 300 by an RNG-RSP message in step 327. If the BS 350 determines that the compensation is required for the MS 300, it transmits an RNG-RSP message with Ranging Status set to 1 to the MS 300.
Upon receipt of the RNG-RSP message with Ranging Status set to 1, the MS 300 transmits another RNG-REQ message to the BS 350, considering that the periodic ranging is still going on in step 329. RNG-REQ/RNG-RSP exchanges take place in steps 331 and 333 in the same manner as in steps 325 and 327, so their detailed description is not provided herein.
If the BS 350 determines that no further frequency, time and transmit power compensation is needed for the MS 300 during the periodic ranging, that is, the periodic ranging is to be completed, it transmits an RNG-RSP message with Ranging Status set to 3 and a Next Periodic Ranging parameter to the MS 300 in step 335.
Upon receipt of the RNG-RSP message, the MS 300 is aware that the periodic ranging 321 has been completed. If the MS 300 is still in a sleep interval, it transitions to the awake mode in step 337.
In the mean time, the MS 300 transitions to the awake mode in a frame indicated by the Next Periodic Ranging parameter set in the RNG-RSP message in step 339. As described earlier, if the MS 300 is in the awake mode, it just performs periodic ranging in the frame indicated by the Next Periodic Ranging parameter. After steps 339 to 343, the periodic ranging is performed in the above-described manner and thus will not be repeated.
As described above with reference to FIGS. 2 and 3, the periodic ranging proposed for the conventional IEEE 802.16e communication system is viable only when both the MS and the BS are normal. In other words, if either of the MS and the BS is in an abnormal state, the periodic ranging is impossible.
Accordingly, there exists a need for a novel periodic ranging scheme that considers when either of the MS and the BS is placed in an abnormal state.